1. Field of the Disclosure
This invention is related to controllers for power supplies. In particular, the invention is related to controllers that control two parameters of a switching signal that controls a switch in a switching power supply.
2. Background
In a typical application, an ac-dc power supply receives an input that is between 100 and 240 volts rms (root mean square) from an ordinary ac electrical outlet. Switches in the power supply are switched on and off by a control circuit to provide a regulated output that may be suitable for providing current to light emitting diodes (LEDs) for illumination. The output is typically a regulated dc current, and the voltage at the LEDs is typically less than 40 volts.
An ac-dc power supply that provides regulated current to LEDs typically must meet requirements for power factor and efficiency as explained below. Designers are challenged to provide satisfactory solutions at the lowest cost.
The electrical outlet provides an ac voltage that has a waveform conforming to standards of magnitude, frequency, and harmonic content. The ac current drawn from the outlet, however, is determined by the characteristics of the power supply that receives the ac voltage. In many applications, regulatory agencies set standards for particular characteristics of the current that may be drawn from the ac electrical outlet. For example, a standard may set limits on the magnitudes of specific frequency components of the ac current. In another example, a standard may limit the rms value of the current in accordance with the amount of power that the outlet provides. Power in this context is the rate at which energy is consumed, typically measured in the units of watts.
The general goal of all such standards for the ac current is to reduce the burden on the system that distributes ac power, sometimes called the power grid. Components of the current at frequencies other than the fundamental frequency of the ac voltage, sometimes called harmonic components, do no useful work, but yet the power grid must have the capacity to provide them and it must endure losses associated with them. Harmonic components generally distort the ideal current waveform so that it has a much higher maximum value than is necessary to deliver the required power. If the power grid does not have the capacity to provide the harmonic components, the waveform of the voltage will drop to an unacceptable value at times that are coincident with the peaks of the distorted waveform of the current. The most desirable ac current has a single frequency component that is at the fundamental frequency of the ac voltage. Moreover, the waveform of the most desirable ac current will be in phase with the ac voltage. That is, the peak ac current will occur at the same time as the peak of the ac voltage. The ideal current will have an rms value that is equal to the value of the power from the outlet divided by the rms value of the voltage. In other words, the product of the rms voltage and the rms current will be equal to the power from the outlet when the current has ideal characteristics.
Power factor is a measure of how closely the ac current approaches the ideal. The power factor is simply the power from the outlet divided by the product of the rms current multiplied by the rms voltage. A power factor of 100% is ideal. Currents that have frequency components other than the fundamental frequency of the ac voltage will yield a power factor less than 100% because such components increase the rms value but they do not contribute to the output power. Currents that have only the fundamental frequency of the ac voltage but are not in phase with the ac voltage will also yield a power factor less than 100% because the power from the outlet is reduced when the peak ac current does not occur at the same time as the peak ac voltage, while the rms value of the current remains at its ideal minimum value. That is, an ideal ac current that is not in phase with the ac voltage will yield a power factor less than 100%. The fundamental frequency of the ac voltage is typically either 50 Hz or 60 Hz in different regions of the world. By way of example, the fundamental frequency of the ac voltage is nominally 60 Hz in North America and Taiwan, but it is 50 Hz in Europe and China.
Since the power supply that receives the ac voltage determines the characteristics of the ac current, power supplies often use either special active circuits or special control techniques to maintain a high power factor. Power supplies that use only ordinary passive rectifier circuits at their inputs typically have low power factors that in some examples are less than 50%, whereas a power factor substantially greater than 90% is typically required to meet the standards for input current, such as for example the International Electrotechnical Commission (IEC) standard IED 61000-3-2. Although regulatory agencies in some regions may impose the standards, manufacturers of consumer equipment often voluntarily design their products to meet or to exceed standards for power factor to achieve a competitive advantage. Therefore, ac-dc power supplies for LEDs, for example, typically must include power factor correction.
The efficiency of a power supply is a measure of how much of the power received by the power supply is delivered to the output of the power supply. A power supply that is 100% efficient delivers to the output all the power it receives at the input. A power supply that is for example 80% efficient delivers only 80% of the power it receives to the output, losing 20% of the power it receives. Regulatory agencies usually mandate minimum efficiencies for power supplies under various operating conditions. The efficiency of a power supply usually has a strong relationship to the switching frequency. Therefore, power supplies typically must control the switching frequency to maintain high efficiency.
To provide a regulated output current at high efficiency from a power factor corrected ac input, a power supply typically varies both the on-time and the frequency of a switching signal that switches a switch. As such, there is a need for an integrated circuit controller that can vary two control parameters in a precise and coordinated manner at low cost.